Relaxation of vibrational energy in fullerene suspensions

Picosecond transient absorption is used to measure the thermal decay time of a mixture of higher-order fullerenes suspended in chloroform, toluene, and carbon disulfide. The relaxation time is 40 ps, which translates into a small effective interface conductance, G 14 MWm 2 K . Molecular dynamics simulations of a fullerene molecule suspended in octane agree with experiment (G 10 MWm 2 K ) and support the conclusion that the 40 ps decay time results from the relaxation of vibrational energy and not the relaxation of electronic excitations. 2005 Elsevier B.V. All rights reserved. The relaxation of photo-excited fullerene molecules has been the subject of considerable research in recent years. Time-resolved pump–probe optical measurements have been performed on C60 thin films [1–5] and toluene suspensions of C60 [6–10], C70 [6], and C76 and C84 [11]. Relaxation times on the order of 40 ps have been observed in many of these experiments [1–4,6,11] and, while all of the authors have attributed the relaxation to a decay or transport property of the electronic system, the authors disagree on the underlying mechanism. We measured the near-infrared transient absorption of a mixture of higher-order fullerenes suspended in toluene, chloroform, and carbon disulfide and argue that the time-evolution of the optical absorption is a signature of the vibrational relaxation of fullerene molecules, not the decay of electronic excited states. Our speculation that the origin of the relaxation is vibrational is supported by our classical molecular dynamics simulations 0009-2614/$ see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.03.052 * Corresponding author. Present address: Department of Mechanical Engineering, Virginia Polytechnic Institute and State University, 114U Randolph Hall, Blacksburg, VA 24061, United States. Fax: +1 540 231 9100. E-mail address: [email protected] (S.T. Huxtable). that do not include electronic effects, yet give a relaxation time comparable to the experimentally measured value. The relaxation of vibrational modes of small molecules in condensed phases has been the subject of research for at least 30 years [12–21]. The relaxation of photo-excited fullerenes provides a useful bridge between the extensive literature on vibrational energy transport in small molecules and more recent studies of interfacial heat transport in nanoparticle [22] and nanotube suspensions [23]. We believe that there are two components to the observed decay. The absorption of a photon excites high frequency vibrational modes within the fullerene which do not couple efficiently with the surrounding fluid. Therefore, the energy associated with these high frequency modes must first be transferred into low frequency vibrational modes before the energy can finally be transmitted to the solvent. Since the entire energy transfer process includes both the cascade of vibrational energy within the fullerene as well as the energy transfer across the interface to the surrounding liquid, the decay is not solely a property of the interface. In fact, we will later argue that the cascade of energy from high frequency modes to low frequency modes may be the